Apparatus for converting noxious pollutants from flue gas into merchantable by-products

ABSTRACT

A system and method of converting flue gas pollutants to marketable byproducts of ammonium bisulfite and ammonium bisulfate, which are collected, and to other harmless byproducts which may be safely discharged is provided. The system removes as much particulate material and reaction inhibiting coal tar components as possible via an electrostatic precipitator and then passes the flue gas through a heat exchanger, wherein it is cooled by boiler feedwater. A suds producing detergent is introduced to the flue gas in order to separate any remaining coal tar components from the condensing moisture. Due to the removal of the tar components, sulfur dioxide and nitrous oxide readily dissolve in the condensing moisture when sufficiently cooled.

This application is a division of Ser. No. 07/888,931, filed May 26,1992, now U.S. Pat. No. 5,230,870.

FIELD OF THE INVENTION

This invention relates to a system for the removal of noxiouspollutants, including compounds of sulfur and nitrogen, from boilerplant flue gases. It also relates to a system for the production ofammonium bisulfate and ammonium bisulfite.

BACKGROUND OF THE INVENTION

Boiler plants, particularly those which generate heat through thecombustion of coal, are well known sources of air and water pollution.The emissions of sulfur from these plants have been well-documented as amajor contributor to the problem of "acid rain". Various toxic orotherwise undesirable compounds are also produced during the combustionof coal, these including uranium, beryllium, chromium, barium, arsenic,selenium, mercury and coal tar.

The worsening environmental impact of the above described pollutants hasbecome increasingly apparent in the last decade. Thus, considerableresources have been expended to come up with reliable andenvironmentally acceptable ways of removing the pollutants from flue gasemissions.

The currently available techniques for removing these pollutants includecooling the flue gas down to condensation within a heat exchanger thatheats boiler feedwater. The prior art, for example, recognizes thatcooling flue gases that contain sulfur trioxide and water vapor willresult in condensation of sulfuric acid. See U.S. Pat. No. 4,526,112 toWarner; U.S. Pat. No. 4,874,585 to Johnson et al; and U.S. Pat. No.4,910,011 to Dorr et al.

Although the aforementioned systems are more or less effective inremoving some degree of the pollutants from the flue gas, they merelyshift the pollution category from atmospheric waste to solid waste.Attempts have therefore been made to convert pollutants into usefulproducts as they are removed from the flue gases.

One such approach involves the introduction of ammonia to combine withsulfur oxides in flue gases and form ammonium bisulfite and/or ammoniumbisulfate.

This approach fails to address the presence of other pollutants in theflue gas, such as nitrogen compounds, uranium, beryllium, chromium,barium, arsenic, selenium, mercury and coal tar. Further, optimumtransfer of heat to the boiler feedwater requires the utilization ofdensely packed tubes through the heat exchanger. Thus, the approach asdescribed above is inefficient and commercially impracticable becausethe dry products produced thereby have a tendency to buildup on thetubes and to severely impede the flow of flue gas.

It has been further recognized in the prior art that SO₂ is not readilyoxidized to SO₃ despite the presence of sufficient atmospheric oxygen inthe flue gas to react therewith. In response to this recognition, theprior art has introduced oxidizing agents such as hydrogen peroxide (H₂O₂) dissolved in water in order to precipitate H₂ SO₄ from the sulfurdioxide within the heat exchanger. See, for example, U.S. Pat. No.4,783,326 to Srednicki. However, the added cost of introducing therequired amount of oxidizing agents make such systems economicallyunattractive as well as complex.

It is therefore an object of the present invention to provide a systemfor economically removing pollutants from flue gas by whichsubstantially all oxides of sulfur and nitrogen, as well as otherharmful contaminants are removed.

It is a further object of the invention to provide such a system inwhich the oxides of sulfur and nitrogen are not merely removed from theflue gas to be disposed elsewhere, but are converted into useful andenvironmentally safe substances, particularly ammonium sulfate, ammoniumbisulfate, and ammonium sulfite.

It is another object of the present invention to provide such a systemin which the formation of the useful byproducts does not hamper theproper functioning of the apparatus.

SUMMARY OF THE INVENTION

These and other objects of the present invention that would be apparentto one skilled in the art are provided by the present invention, whichcomprises a heat exchanger located along a duct containing flue gas toexhaust, means for introducing an ammoniacal substance to said flue gas,means for introducing a suds producing detergent to the flue gas, meansfor collecting ammonium bisulfate and ammonium bisulfite formed by thereaction of the ammoniacal substance with oxides of sulfur in the fluegas, and means for removing particulate material from the flue gasbefore it enters the heat exchanger.

Boiler feedwater is delivered to the heat exchanger after passingthrough a condenser. The boiler feedwater moves upwardly through theheat exchanger through either acid-proof copper alloy pipes or betweenclosely spaced, acid-resistant roll-bonded sheets having serpentinecircuits therebetween.

According to another aspect of the invention, a method of removingpollutants from the flue gases includes exchanging heat between flue gasand condensed boiler feedwater to cool the flue gas down to a firsttemperature whereat substantially all SO₃ in the flue gas is combinedwith H₂ O, condensing the SO₃ and H₂ O from the flue gas as a firstcondensate, adding a solution containing an ammoniacal substance and asuds producing detergent to the flue gas, collecting the soap sudsproduced after the adding step and the first condensate as a firstsolution, and separating ammonium bisulfate from the first solution.

The method further includes the steps of exchanging heat between saidflue gas and cooling water at a precisely controlled temperature to coolthe flue gas down to a second temperature, condensing H₂ O from the fluegas as a second condensate, dissolving SO₂ in the flue gas into thesecond condensate; and collecting the second condensate and part of theammoniacal substance introduced during said addition step to form asecond solution containing ammonium bisulfite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the arrangement of the components of thepresent invention along the flows of flue gas and boiler feedwater in atypical power plant application;

FIG. 2 is a humidity chart showing the LBS of water per LB of "bone dry"flue gas.

FIG. 3 is a cut-away view of the heat exchanger showing a arrangement ofroll-bonded sheets which may be used to carry boiler feedwater throughthe heat exchanger.

FIG. 4 is a detail of the end of the tubes which may be used in the heatexchanger in lieu of roll-bonded sheets.

FIG. 5 is a cutaway view showing the connection between the tubes of thearrangement in FIG. 4.

FIG. 6 is a diagrammatic flowsheet showing the arrangement of the heatexchanger, recirculating air lift, and mineral flotation and filteringsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The flow diagram of FIG. 1 shows the basic configuration of theinvention within a conventional power plant system. Fossil fuel 2 issupplied to a boiler 4 which produces steam. The steam drives turbines6, and a condenser 8 then cools the steam, returning it to the boilerfeedwater state. The condensed feedwater 10 exits the condenser at about40° F. to 80° F. Some remaining wet steam 12 is used in a preheater 14for the boiler feedwater, which is returned to the boiler via pumps 16.

Flue exhaust gas 18 leaving the boiler is first cooled from about 700°F. to about 600° F. by passing it through an air preheater 20.Preheating the boiler combustion air increases the efficiency of boilercombustion.

The flue gas is next directed through an electrostatic precipitator orbaghouse 24 to remove particulates 26, such as fly ash. This serves twopurposes. First, by removing them prior to introduction of the flue gasinto the heat exchanger, the particulates are prevented fromaccumulating on the heat exchanging surfaces and thus from interferingwith heat transfer processes. Second, the adverse effects of particulatepollutants on the desired reactions taking place within the heatexchanger are minimized.

As a matter of chemistry, it has long been known that the presence ofcertain substances can have a "negative catalytic effect" on reactionsbetween other substances. Fly ash and other particulates in the flue gascontain coal tar components such as phenol, which can act as a negativecatalyst to the reactions sought by the present invention. For example,one of the reactions sought by this invention and impeded by thepresence of coal tar components is the reaction of SO₂ in the flue gaswith ammonium hydroxide.

For the above reasons, the baghouse 24 is placed ahead of the heatexchanger 26 to minimize the amount of the coal tar derivatives presentin the flue gas. After the flue gas passes through baghouse 24, whereinit may cool to a temperature of approximately 700° F., the flue gas isdrawn downwardly at approximately 40 feet per second through a verticalheat exchanger 26 by a conventional induced draft fan, not shown. Heatis exchanged therein between the flue gas and boiler feedwater therebyreducing the amount of fuel needed to heat the feedwater.

As best shown in FIG. 3, the heat exchanger is preferably comprised of aacid proof housing, preferably formed of plastic, and a vertical columnof consecutive, spaced layers of acid proof fluid conveying meanscarrying boiler feedwater therethrough.

Preferably, the fluid conveying means are parallel, horizontally spacedsheets 23 of stainless steel or other corrosion resistant metal having aserpentine circuit provided between adjacent sheets. The serpentinecircuits are formed by a plurality of vertically spaced, horizontallyextending raised portions or corrugations 21 on the surfaces of thesheets 23. These raised portions are preferably formed on the sheets byrollbonding, although any conventional method of producing spacedprojections on a sheet of metal may be used.

The sheets are preferably arranged so that respective raised portions 21of one sheet are positioned opposite to corresponding flat portions ofthe sheet facing it. The alternating "washboard" arrangement of raisedportions and flat portions on each sheet results in a zig-zag flowpattern which greatly increases the heat transfer rate. Each serpentinecircuit includes an inlet 23a, where cool water enters the heatexchanger, and an outlet 23b where hot water exits the heat exchanger.

Alternatively, and as illustrated in FIGS. 4 and 5, vertically spaced,horizontally extending tubes 72 may be used in lieu of the roll bondedsheets. Preferably, tubes 72 are constructed of a nylon coated coppernickel alloy. However, other metal or metal alloys having high thermalconductivity and suitable corrosion protection may also be used. Theends of tubes 72 are connected by a suitable bend as shown in FIG. 5.

As shown in FIG. 6, the layers of heat exchanging pipes or rollbondedsheets are arranged in three sections 30, 40, and 50. The sections areseparated from each other by inverted pyramid shaped pans 31 and 41,each of the pans serving to collect and funnel corresponding condensatesolutions which precipitate at various temperatures within the heatexchanger. The condensate solution flowing through conduit 32 collectedby the first pan 31 contains both noxious impurities as well asmarketable ammonium sulfate. The condensate solution flowing throughconduit 45 collected by the second pan 41 contains ammonium sulfite,herewith converted to ammonium bisulfate. Lastly, a third pan 51 formsthe base of the heat exchanger and collects purified condensate 42 fromthe flue gas for disposal in streams, lakes, or the ocean. Preferably,these pans are made of plastic and have deep, downwardly sloping groovesformed on their upper surfaces to carry off condensates from highvelocity flue gas.

Even after it passes through the baghouse 24, I have found that the fluegas 18 still contains significant quantities of phenol and othercompounds. Further, I have discovered that these trace quantities form acoating on each and every drop of condensing moisture and that thiscoating resists the dissolution of SO₂ into the condensate.

The present invention ensures that the SO₂ contained in the flue gas isdissolved in the moisture of condensation by means which will beexplained below. According to the invention, solid ammonium bisulfate orammonium sulfite, useful as fertilizer, is then formed by theintroduction of a sufficient amount of either ammonia or ammoniumhydroxide, and a suds-producing detergent or soap into the flue gas atcertain specified locations.

The operation of the present invention may be best understood byreference to FIG. 6. As the flue gas is cooled in the heat exchanger 26,H₂ SO₄ is produced by the reaction of the SO₃ with H₂ O. These reactionsbegin at high temperatures with SO₃ producing H₂ SO₄ with no moleculesof H₂ O at 626° F., 1 molecule of water at 554° F., 2 molecules of H₂ Oat 333° F., and 4 molecules of H₂ O at 250° F.

When the flue gas has been cooled to a temperature below 212° F. butstill above the point of condensation of moisture (which will varydepending upon the fuel used), fine sprays 17 of ammonium hydroxide andsuds-producing detergent or soap are injected, via line 13, underpressure from pump 15 into the flue gas from locations around the acidproof plastic enclosure housing the heat exchanger. These sprays areadded just before the first moisture condenses within the flue gas,preferably at a point within 2° F. from the moisture condensation point.

It is important that ammonia not be added at a temperature too muchabove the moisture condensation point because crusts of ammonium sulfatemay form on the walls of the chamber or between the closely spacedlayers of acid-proof heat exchanger tubes or rollbonded sheets whichcontain the condenser feedwater. To achieve precise regulation of theflue gas temperature, cooling water 11 having a precisely controlledinlet temperature is preferably introduced into the heat exchanger. Thecooling water may be drawn from a conventional tank or reservoir 19 andintroduced by several layers of acid proof pipes or roll bonded sheetsat a point just following the level where NH₄ OH is injected,

The SO₃ reports in the first condensate as H₂ SO₄, which in turn reactswith some of the injected ammonium hydroxide to form ammonium bisulfate.A solution 32 containing ammonium bisulfate, soap suds, and other tracepollutants collects in first pan 31 and drains into a conventionalmineral flotation machine 34. The flotation machine 34 has a pluralityof mineral flotation cells 35. The soot and other coal tar componentscollected with the first solution 32 is floated to the surface of thefloatation cells 35 via tiny soap bubbles and moves from right to leftby skimmers 36 which rotate clockwise. The "pulp" or liquid solution ofsoluble ammonium sulfates and insolubles which are suspended thereinmoves from right to left through the cells and into a V-shaped tank 37.Waste, soot, and insoluble pollutants are separated and removed from theliquid solution by a vacuum filter 43, whereafter a purified solution ofsoluble ammonium sulfates 44 may be removed, separated by differentialcrystallization, and then marketed as crop fertilizer or the like.

Referring now to FIG. 2, it is possible to determine how much H₂ O willcondense from the flue gas as it is cooled. Burning 1 lb of coalnormally produces approximately 10 lbs of flue gas. Using the chart ofFIG. 2, cooling 10 lbs of flue gas from 120° F. to 116° F. will condense1.2 lbs of H₂ O. At approximately 115° F., the SO₂ in the flue gas willdissolve into the soot-free condensing moisture, whereafter it reactswith ammonium hydroxide to produce ammonium sulfite in accordance withthe following formula:

    SO.sub.2 +NH.sub.4 OH→NH.sub.4 HSO.sub.3

The resulting condensate containing the ammonium sulfite collects on thesecond pan 41 for removal and conversion into ammonium bisulfate as willbe described below.

Although the present invention is capable of dissolving all of the SO₂into the H₂ O and converting it to (NH₄)₂ HSO₃ (ammonium bisulfite), itmay be necessary to supply more water than that provided bycondensation. Accordingly, sufficient water may be included in theammonium hydroxide and detergent sprays, if production of ammoniumbisulfite, instead of ammonium sulfite, is desired at this stage.

The collected ammonium sulfite drains from the second pan 41 and isintroduced into an inlet 61 of a conventional recirculating Pohl airlift 60. Preferably, the recirculating lift 60 comprises a cylindricaltank 62 having an approximate height of 30 feet and an approximatediameter of 6 feet axially aligned within a Pohl air lift pipe 63 havinga diameter of approximately 10 feet. Ammonia (NH₃) and atmosphericoxygen O₂ are introduced through an inlet 64 in the bottom of the tank62. The lift evolves moist air, which exits through an outlet 65proximate the top of the lift pipe 63, thus concentrating the solutioncontained therein. The NH₃, O₂, and NH₂ HSO₄ react in the recirculatinglift in accordance with the following equation:

    2NH.sub.4 HSO.sub.3 +2NH.sub.3 +O.sub.2 →2(NH.sub.4).sub.2 SO.sub.4

The ammonium bisulfate produced is collected and removed from the liftat arrow 66, whereupon it may be marketed as a crop fertilizer aspreviously described.

In some cases, it is recognized that a power plant may be unable toobtain boiler feed water cold enough to cool the flue gas below 120° F.Here again the present invention advantageously utilizes the injectionof fine sprays of water into the heat exchanger in order to cool theflue gas by the heat of evaporation. For example, 0.01 lbs of added H₂ Ocools the flue gas by the heat of evaporation, which in this case equals10 BTU/0.29 (specific heat of flue gas) or 34° F. Flue gas at atemperature of 130° F., for example, may thus be cooled to 96° F. byinjecting additional water. The injected cooling water must be at 86° F.or less in order to cool the flue gas to 96° F. in the manner describedabove.

Preferably the action of the heat exchanger, supplemented if needed byadditional water injection, cools the flue gas to approximately 50° F.

Because the cooled flue gas has already been cleaned of soot and coaltar derivatives, the NO present therein will now readily dissolve in theH₂ O. Once dissolved, the NO reacts with introduced atmospheric oxygenand the remaining ammonium hydroxide according to the followingequation:

    O.sub.2 (AIR)+4NO+4NH.sub.4 OH→4NH.sub.4 NO.sub.2 +2H.sub.2 O

The NH₄ NO₂ then decomposes into N₂ and H₂ O.

The final purified condensate 42 collected in the base pan 51 issubstantially detoxified and can be safely discharged through an outlet52 as waste water from the facility. A set of demisting screens 53 isprovided to catch droplets of moisture before the flue gas exits throughthe exhaust duct 54.

After the pollutants have been removed therefrom in the manner describedabove, the cleaned and cooled flue gas 28 is blown by an induced draftfan through an exit duct 54 by which it is led to a smoke stack 40.

Ordinarily, flue gas would contain more pollutants and would have to bedischarged out of a high stack. However, a high flue gas temperature isnecessary to successfully discharge gas through such stacks, which areoften hundreds of feet high. The purity of the flue gas treated by thepresent invention allows a shorter stack to be used in discharging thegas. This, in turn, obviates the need to maintain the gas at an elevatedtemperature so that additional cooling and heat recovery may beobtained.

What is claimed is:
 1. A heat exchanger and pollutant removal system forfossil fuel boiler plants, comprising:a fossil fuel boiler; an exhauststack; heat exchanger means forming a portion of a duct which directs aflue gas stream from said fossil fuel boiler to said exhaust stack andhaving a flue gas inlet, a flue gas exit, and first fluid delivery meansfor carrying a first fluid and removing heat from the flue gas; a sourceof ammoniacal substance; means on said heat exchanger means, and influid connection with said source of ammoniacal substance, forintroducing said ammoniacal substance into said flue gas; and firstmeans, located below said first fluid delivery means, for collecting andremoving a first solution of soot, part of said ammoniacal substance,and SO₃ dissolved in water, wherein the ammoniacal substance introducingmeans is located upstream of the collection and recovery means.
 2. Thesystem of claim 1 wherein said first fluid delivery means comprises aplurality of acid resistant metal tubes.
 3. The system of claim 2wherein the tubes are made of a copper-nickel alloy having a nyloncoating on the exterior.
 4. The system of claim 1 wherein said firstfluid delivery means comprises a plurality of acid resistant, spaced,sheets having spaced projections thereon, adjacent sheets forming aserpentine circuit therebetween.
 5. The system of claim 4 wherein thesheets are made of stainless steel.
 6. A heat exchanger and pollutantremoval system for fossil fuel boiler plants, comprising:a fossil fuelboiler; an exhaust stack; heat exchanger means forming a portion of aduct which directs a flue gas stream from said fossil fuel boiler tosaid exhaust stack and having a flue gas inlet, a flue gas exit, andfirst fluid delivery means for carrying a first fluid and removing heatfrom the flue gas; a source of ammoniacal substance and a source ofsuds-producing detergent; means on said heat exchanger in fluidconnection with said source of ammoniacal substance for introducing saidammoniacal substance and means for introducing a suds-producingdetergent into said flue gas; first means, located below said firstfluid delivery means, for collecting a first solution of suds, soot,part of said ammoniacal substance, and SO₃ dissolved in water; andmineral flotation means connected to said first means for receiving thefirst solution.
 7. The system of claim 1 wherein said collecting meansincludes means for separating soot from the first solution.
 8. Thesystem of claim 7 wherein the separating means comprise a vacuum filter.9. The system of claim 1, wherein said heat exchanger further comprisessecond fluid delivery means for carrying a second fluid and removingheat from the flue gas, and wherein said system further comprises meansfor maintaining said second fluid at a predetermined temperature priorto delivery into said heat exchanger means.
 10. A heat exchanger andpollutant removal system for fossil fuel boiler plants, comprising:afossil fuel boiler; an exhaust stack; heat exchanger means forming aportion of a duct which directs a flue gas stream from said fossil fuelboiler to said exhaust stack and having a flue gas inlet, a flue gasexit, and first fluid delivery means for carrying a first fluid andremoving heat from the flue gas; a source of ammoniacal substance and asource of suds-producing detergent; means on said heat exchanger meansin fluid connection with source of ammoniacal substance for introducingsaid ammoniacal substance and means in fluid connection with said sourceof suds-producing detergent for introducing said suds-producingdetergent into said flue gas; first means, located below said firstfluid delivery means, for collecting a first solution of suds, soot,part of said ammoniacal substance, and SO₃ dissolved in water; secondfluid delivery means for carrying a second fluid to said heat exchangermeans and removing heat from the flue gas; means for cooling said secondfluid prior to delivery into said heat exchanger means; and second meansfor collecting a second solution containing SO₂ dissolved in condensedwater and part of said ammoniacal substance.
 11. The apparatus of claim10 further comprising means for removing said second solution.
 12. Theapparatus of claim 11 further comprising recirculating means forreceiving said second solution from said removing means and having meansfor recirculating said second solution with an ammoniacal substance andair to produce ammonium bisulfate.
 13. A heat exchanger and pollutantremoval system for fossil fuel boiler plants, comprising:a fossil fuelboiler; an exhaust stack; heat exchanger means forming a portion of aduct which directs a flue gas stream from said fossil fuel boiler tosaid exhaust stack and having a flue gas inlet, a flue gas exit, andfirst fluid delivery means for carrying a first fluid and removing heatfrom the flue gas; a source of ammoniacal substance and a source ofsuds-producing detergent; means on said heat exchanger means in fluidconnection with said source of ammoniacal substance and said source ofsuds-producing detergent for introducing said ammoniacal substance andsuds-producing detergent into said flue gas; and first means, locatedbelow said first fluid delivery means, for collecting a first solutionof suds, soot, part of said ammoniacal substance, and SO₃ dissolved inwater.
 14. The system of claim 13 further comprising means forseparating said suds and soot from said first solution.
 15. The systemof claim 14 wherein the separating means comprises a vacuum filter. 16.The system of claim 1 further comprising means for separating said sootfrom said first solution.
 17. A heat exchanger and pollutant removalsystem for fossil fuel boiler plants, comprising:a fossil fuel boiler;an exhaust stack; heat exchanger means forming a portion of a duct whichdirects a flue gas stream from said fossil fuel boiler to said exhauststack and having a flue gas inlet, a flue gas exit, and first fluiddelivery means for carrying a first fluid and removing heat from theflue gas and second fluid delivery means for carrying a second fluid andfurther removing heat from the flue gas; a source of ammoniacalsubstance; means on said heat exchanger in fluid connection with saidsource of ammoniacal substance for introducing an ammoniacal substanceinto said flue gas; first means, located below said first fluid deliverymeans, for collecting a first solution of part of said ammoniacalsubstance and SO₃ dissolved in water; second means located below saidsecond fluid delivery means and said first solution collection means forcollecting a second solution containing SO₂ dissolved in condensed waterand part of said ammoniacal substance.